Update to roxygen2 @template for ggplot2_args arguments (#694)

# Pull Request

<!--- Replace `#nnn` with your issue link for reference. -->

- Closes #695
- Part of #624

#### Changes description

- Update type in `ggplot2_args`
- Minor change to phrasing
- Move template file under `man` page to avoid extra folder at root
level ([available
since](https://roxygen2.r-lib.org/news/index.html?q=template#options-7-0-0)
`[email protected]`)
- Update `.Rbuildignore` accordingly

#### Reviewer should consider

- `@template` has been superseded
(https://roxygen2.r-lib.org/articles/reuse.html?q=template#superseded)
  - There's no equivalent though _(with parameterized strings)_
- Alternative: we could add `ggplot2_args` to `shared_params` and
specific list of names it can take in `@details` on each function that
uses this

.Rbuildignore

@@ -28,7 +28,7 @@ SECURITY.md
 ^Jenkinsfile$
 ^logs$
 ^Makefile$
-^man-roxygen$
+^man/roxygen$
 ^Meta$
 ^outputdir$
 ^scratch$

man/roxygen/templates/ggplot2_args_multi.R

@@ -0,0 +1,8 @@
+#' @param ggplot2_args (`ggplot2_args` or `named list` of `ggplot2_args`s) optional.
+#' Object created by [`teal.widgets::ggplot2_args()`] and contains settings for all the plots,
+#' or is a named `list` of `ggplot2_args` objects for plot-specific settings.
+#' The argument is merged with options variable `teal.ggplot2_args` and default module setup.
+#'
+#' List names should match the following: `c("default", <%=ggnames%>)`.
+#'
+#' For more details see the vignette: `vignette("custom-ggplot2-arguments", package = "teal.widgets")`.